Nitride semiconductor laser device and method of producing the same

ABSTRACT

A method of producing a nitride semiconductor laser device includes: forming a wafer including a nitride semiconductor layer of a first conductivity type, an active layer of a nitride semiconductor, a nitride semiconductor layer of a second conductivity type, and an electrode pad for the second conductivity type stacked in this order on a main surface of a conductive substrate and also including stripe-like waveguide structures parallel to the active layer; cutting the wafer to obtain a first type and a second type of laser device chips; and distinguishing between the first type and the second type of chips by automatic image recognition. The first type and the second type of chips are different from each other in position of the stripe-like waveguide structure with respect to a width direction of each chip and also in area ratio of the electrode pad to the main surface of the substrate.

PRIORITY STATEMENT

This application is a divisional under 35 U.S.C. §121 of U.S.application Ser. No. 11/715,443, filed Mar. 8, 2007, which claimspriority under 35 U.S.C. §119 to Japanese Application No. 2006-069350,filed Mar. 14, 2006, the entire contents of each of which are herebyincorporated by reference.

BACKGROUND

1. Technical Field

The present invention relates to improvement in a nitride semiconductorlaser device and a method of producing the same.

2. Description of the Related Art

From the past, prototypes of nitride semiconductor laser devices foremitting light in a wavelength range from blue to ultraviolet have beenfabricated using nitride-based semiconductor materials such as GaN, InN,AlN, and a mixed crystal semiconductor thereof.

FIG. 9 is a schematic cross-sectional view exemplarily showing a mainpart of a conventional nitride semiconductor laser device for lasing ata wavelength of 405 nm. This semiconductor laser device includes ann-type GaN layer (thickness: 3 μm) 902; an n-type In_(0.05)Ga_(0.95)Nbuffer layer 903; an n-type Al_(0.05)Ga_(0.95)N clad layer (thickness:2.0 μm) 904; an n-type GaN optical waveguide layer (thickness: 0.1 μm)905; an In_(0.2)Ga_(0.8)N/n-type In_(0.05)Ga_(0.95)N (respectivethicknesses: 4 nm/8 nm) triple-quantum-well (3MQW) active layer 906; ap-type Al_(0.2)Ga_(0.8)N carrier stop layer (thickness: 20 nm) 907; ap-type GaN optical waveguide layer (thickness: 0.1 μm) 908; a p-typeAl_(0.05)Ga_(0.95)N clad layer (thickness: 0.5 μm) 909; and a p-type GaNcontact layer (thickness: 0.2 μm) 910 stacked in this order on an n-typeGaN substrate 901.

A ridge-like stripe 902 of 2 μm width is formed by partially etchingp-type GaN contact layer 910 through to a partial depth of p-type AlGaNclad layer 909. Accordingly, the laser device of FIG. 9 has an opticalconfinement waveguide structure in which active layer 906 and opticalwaveguide layers 905 and 908 are sandwiched between clad layers 904 and909, and then light generated in active layer 906 is confined in thiswaveguide structure and causes lasing action. It should be noted thatFIG. 9 does not show the entire width of the laser device chip but onlya part thereof in the vicinity of the ridge-like stripe.

A p-side contact electrode 911 is formed on p-type GaN contact layer 910left after the partial etching, and insulating films 913 are formed onthe partially etched areas. A p-side electrode pad 914 is formed incontact with p-side contact electrode 911. On the other hand, an n-sideelectrode 915 is formed on a rear surface of substrate 901.

The laser device of FIG. 9 is obtained by dividing a wafer including alarge number of laser device structures into chips. To facilitate thechip division, therefore, substrate 901 is polished to have a thicknessof 50 to 200 μm before n-side electrode 915 is formed thereon. Then, thenitride semiconductor laser device of FIG. 9 with a width of 300 to 400μm is obtained by dividing the wafer into chips.

FIG. 10 is a schematic plan view of the nitride semiconductor laserdevice of FIG. 9 seen from above. Ridge-like stripe 912 is formed on theupper surface of the nitride semiconductor laser device, and the p-sidecontact electrode (not shown) is formed over the entire upper surface ofthe ridge-like stripe. Further, p-side electrode pad 914 is formed tocover almost the entire upper surface of the nitride semiconductor laserdevice.

The nitride semiconductor laser device of FIG. 10 is mounted on a stem,and thereafter wire bonding is performed in an area not includingridge-like stripe 912 on p-side electrode pad 914. Therefore, p-sideelectrode pad 914 is generally formed such that a circular region ofmore than 80 μm diameter is securely obtained for a wire bonding area.

Generally, in order to reduce the cost for a nitride semiconductor laserdevice, it is desirable to increase the yield rate of the laser devicesobtained from one wafer by reducing the width of the laser device to 50to 250 μm. However, when a nitride semiconductor laser device has areduced width, problems arise in a laser device fabricated by atechnique described below.

In Japanese Patent Laying-Open No. 2004-356454, trenched regions eachhaving a stripe-like groove are formed at an interval of several hundredmicrometers on a nitride semiconductor substrate in order to suppressoccurrence of cracks during crystal growth of nitride semiconductorlayers. Accordingly, a hill portion is naturally formed between adjacenttrenched regions. By growing nitride semiconductor layers and form alayered structure thereof on a substrate subjected to such working asabove (hereinafter referred to as a worked substrate), occurrence ofcracks in the layered structure can be prevented, and surface flatnessof the layered structure over the hill portion can be improved to someextent.

When the nitride semiconductor layers are grown to form the layeredstructure on the worked substrate, however, a swelling with a height ofseveral micrometers is caused adjacent to a trenched region in an uppersurface of the layered structure. Taking account of thephotolithographic process in fabricating a nitride semiconductor laserdevice, therefore, the ridge-like stripe structure should be formed morethan 90 μm away and preferably more than 110 μm away from the trenchedregion.

FIG. 11 is a schematic plan view showing an example of an upper surfaceof a bar obtained after formation of resonator end faces of each nitridesemiconductor laser device (i.e., after a wafer including a large numberof laser device structures is divided into bars) and just before the bar(hereinafter referred to as a laser bar) is divided into individualchips. In FIG. 11, trenched regions 1101 are formed at an interval of800 μm for example, and then ridge-like stripes 1102 and p-sideelectrode pads 1103 are formed between trenched regions 1101. Everydotted line in FIG. 11 indicates a chip division plane, and the chipwidth is set to 200 μm for example.

As described above, in FIG. 11, a distance (L) between ridge-like stripe1102 and trenched region 1101 should desirably be set to more than 110μm. In order to set the chip width to 200 μm, therefore, ridge-likestripe 1102 in each of certain particular chips is set at a positiondifferent from that in the other chips, with respect to the widthdirection of the chip. More specifically, in FIG. 11, the position ofridge-like stripe 1102 in a laser device chip (B) is different from thatin a laser device chip (A), with respect to the width direction of thechip.

The laser bar in such a state as above may cause problems as describedbelow during the subsequent process of producing laser devices.

Firstly, after the laser bar is divided into individual laser devicechips, it is necessary to inspect all the chips. In the case of using anautomatic chip inspection apparatus, if the apparatus is set todetermine the position of the ridge-like stripe in chip (A) as normal,then there is a problem that it determines all of chips (B) asdefective.

Further, when a chip is mounted on a stem, the light-emitting point inthe chip should be made coincident with the center of the stem. However,since the light-emitting point in chip (B) is different from that inchip (A) with respect to the width direction, it is necessary to mountchip (B) at a chip position changed relatively on the stem.

SUMMARY

The present invention has been made to solve the aforementionedproblems, and makes it possible that, when nitride semiconductor laserdevices of a small width are handled in an automatic chip inspectionapparatus or an automatic mounting apparatus, only some particular chipscan be selected readily and automatically by image recognition.

In the specification of the present application, the automatic imagerecognition includes processes of measuring a size of an area having alight intensity of more than a preset threshold value in a lightintensity distribution obtained by imaging a chip from above with acamera, comparing the measured size with a preset size, and determiningwhether it falls within a preset tolerance.

A method of producing a nitride semiconductor laser device according tothe present invention includes the steps of: forming a wafer including anitride semiconductor layer of a first conductivity type, an activelayer of a nitride semiconductor, a nitride semiconductor layer of asecond conductivity type, and an electrode pad for the secondconductivity type stacked in this order on a main surface of aconductive substrate and also including a plurality of stripe-likewaveguide structures parallel to the active layer; cutting the wafer toobtain a first type of nitride semiconductor laser device chip and asecond type of nitride semiconductor laser device chip; anddistinguishing between the first type and the second type of laserdevice chips by automatic image recognition, wherein the first type andthe second type of laser device chips are different from each other inposition of the stripe-like waveguide structure with respect to a widthdirection of each chip and also in area ratio of the electrode pad forthe second conductivity type to the main surface of the substrate.

It is desirable that the area ratio of the electrode pad for the secondconductivity type in the first type of laser device chip is less than90% of that of the electrode pad for the second conductivity type in thesecond type of laser device chip. In each of the first type and thesecond type of laser device chips, a metal layer of less than 0.1 μmthickness may be formed in an area not having the electrode pad for thesecond conductivity type on an upper surface of the semiconductor layerof the second conductivity type, or the metal layer may be omitted.

It is desirable that, in each of the first type and the second type oflaser device chips, the electrode pad for the second conductivity typeincludes a circular area of 80 μm diameter. Preferably, the circulararea is spaced by a distance of more than 10 μm from the stripe-likewaveguide structure.

Preferably, in each of the first type and the second type of laserdevice chips, the main surface of the substrate is a rectangle havingtwo sides parallel to and the other two sides perpendicular to thestripe-like waveguide structure, and each of the other two sides has alength of more than 50 μm and less than 250 μm.

It is desirable that, in each of the first type and the second type oflaser device chips, an area not having the electrode pad for the secondconductivity type on an upper surface of the semiconductor layer of thesecond conductivity type is lower in reflectance by more than 10% withrespect to almost the entire wavelength range of incident illuminationlight vertical to the chip as compared to the other area having theelectrode pad for the second conductivity type during the automaticimage recognition.

For that purpose, it is preferable that the area not having theelectrode pad for the second conductivity type on the upper surface ofthe semiconductor layer of the second conductivity type has a layer ofmore than 10 nm thickness made of an absorbing material having anabsorption coefficient of more than 10000 cm⁻¹ with respect to almostthe entire wavelength range of incident illumination light vertical tothe chip during the automatic image recognition. Preferably, theabsorbing material is insulator or semiconductor, and can includesilicon, germanium, or TiO₂.

Instead of having such a light absorbing layer, the area not having theelectrode pad for the second conductivity type on the upper surface ofthe semiconductor layer of the second conductivity type may have minutesurface unevenness, and a root mean square roughness over a length of 5μm in a direction parallel to the uneven surface is preferably more than1 nm and less than 200 nm.

A nitride semiconductor laser device according to the present inventionincludes a nitride semiconductor layer of a first conductivity type, anactive layer of a nitride semiconductor, a nitride semiconductor layerof a second conductivity type, and an electrode pad for the secondconductivity type stacked in this order on a main surface of aconductive substrate, wherein a stripe-like waveguide structure parallelto the active layer is formed, and an area of the electrode pad for thesecond conductivity type is more than 2% and less than 90% of an area ofthe main surface of the substrate.

In an area not having the electrode pad for the second conductivity typeon an upper surface of the semiconductor layer of the secondconductivity type, a metal layer of less than 0.1 μm thickness may beformed, or may be omitted.

Preferably, the electrode pad for the second conductivity type includesa circular area of 80 μm diameter, and the circular area is spaced by adistance of more than 10 μm from the stripe-like waveguide structure.

Preferably, the main surface of the substrate is a rectangle having twosides parallel to and the other two sides perpendicular to thestripe-like waveguide structure, and each of the other two sides has alength of more than 50 μm and less than 250 μm.

It is desirable that an area not having the electrode pad for the secondconductivity type on an upper surface of the semiconductor layer of thesecond conductivity type is lower by more than 10% in reflectance withrespect to almost the entire wavelength range of incident illuminationlight vertical to the chip as compared to the other area having theelectrode pad for the second conductivity type during the automaticimage recognition.

For that purpose, it is preferable that the area not having theelectrode pad for the second conductivity type on the upper surface ofthe semiconductor layer of the second conductivity type has a layer ofmore than 10 nm thickness made of an absorbing material having anabsorption coefficient of more than 10000 cm⁻¹ with respect to almostthe entire wavelength range of incident illumination light vertical tothe chip during the automatic image recognition. Preferably, theabsorbing material is insulator or semiconductor, and can includesilicon, germanium, or TiO₂.

Instead of having such a light absorbing layer, the area not having theelectrode pad for the second conductivity type on the upper surface ofthe semiconductor layer of the second conductivity type may have minutesurface unevenness, and a root mean square roughness over a length of 5μm in a direction parallel to the uneven surface is preferably more than1 nm and less than 200 nm.

The nitride semiconductor laser device according to the presentinvention can preferably be included as a light source in an opticalinformation reproducing apparatus.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view showing nitride semiconductor laserdevices in a state of a laser bar in accordance with an embodiment ofthe present invention.

FIG. 2 is a schematic plan view illustrating a shape of a p-sideelectrode pad of a chip (B) in FIG. 1 in more detail.

FIG. 3 is a schematic plan view illustrating a shape of a p-sideelectrode pad of a chip (A) in FIG. 1 in more detail.

FIG. 4 is a conceptual block diagram showing a general constitution ofan automatic chip inspection apparatus.

FIG. 5 is a schematic plan view illustrating a shape of a p-sideelectrode pad of a chip regarding a nitride semiconductor laser deviceaccording to another embodiment of the present invention.

FIG. 6 is a schematic cross-sectional view showing a main part of alayered structure in a nitride semiconductor laser device according tostill another embodiment of the present invention.

FIG. 7 is a schematic block diagram illustrating an optical system and apropagation path of light for reproduction in an optical informationreproducing apparatus according to still another embodiment of thepresent invention.

FIG. 8 is a schematic cross-sectional view showing a main part of alayered structure in the nitride semiconductor laser device shown inFIG. 1.

FIG. 9 is a schematic cross-sectional view exemplarily showing a mainpart of a conventional nitride semiconductor laser device.

FIG. 10 is a schematic plan view of the nitride semiconductor laserdevice of FIG. 9 seen from above.

FIG. 11 is a schematic plan view exemplarily showing an upper surface ofa laser bar according to the prior art just before division intoindividual chips.

DETAILED DESCRIPTION First Embodiment

FIG. 1 is a schematic plan view showing nitride semiconductor laserdevices in a state of a laser bar according to a first embodiment of thepresent invention. In a laser bar 101 of this drawing, there are formeda plurality of trenched regions 102. Between trenched regions 102, aplurality of ridge-like stripes 103 are formed, and a plurality ofp-side electrode pads 104 are formed to cover ridge-like stripes 103.Every dotted line in FIG. 1 indicates a chip division plane.

In the first embodiment, an area ratio of p-side electrode pad 104 tothe upper surface area (hereinafter referred to as a chip area) of anitride semiconductor laser device (B) is less than 90% differently fromthat of a nitride semiconductor laser device (A).

In an automatic chip inspection apparatus that uses an automatic imagerecognition process, it is preset to determine whether a chip isacceptable or not based on the shape of p-side electrode pad 104 andpreset to determine nitride semiconductor laser device (A) asacceptable, for example. When the nitride semiconductor laser devices ofthe first embodiment are introduced into the automatic chip inspectionapparatus, only chips (A) are extracted for chip inspection, and chips(B) are determined as having an unacceptable shape and left in a chipsupplying area without being inspected. Therefore, it is possible toeliminate a problem that all of the laser devices are subjected to chipinspection and rather many of them are determined as defective in thecase of chip inspection for the conventional nitride semiconductor laserdevices.

FIG. 2 is a schematic plan view illustrating a shape of the p-sideelectrode pad of chip (B) in more detail. In this drawing, a nitridesemiconductor laser device 201 is set to have a chip width 202 of 50 to250 μm (e.g., 200 μm) and a length 203 (corresponding to a resonatorlength) of 300 to 1500 μm (e.g., 600 μm). A trenched region 204 of aworked substrate exists along the right side of laser device 201. Aridge-like stripe 205 is formed at a position spaced by a distance 208of 120 μm from the right side.

A p-side electrode pad 206 includes a first area covering ridge-likestripe 205 and a second area for wire bonding. The first area has awidth of 5 to 50 μm (e.g., 20 μm) and a length retracted by 5 to 50 μm(e.g., 20 μm) from each end of the resonator. The second area ofelectrode pad 206 is formed as an area (e.g., as a square with a side of80 μm) including a circle 207 of 80 μm diameter.

In chip (B), the area ratio of the p-side electrode pad to the chip areais set to less than 90% of that in chip (A). For example, when the chiphas a width of 200 μm and a length of 600 μm, and the p-side electrodepad includes the first area of 20 μm width and 560 μm length and thesecond area of a square with a side of 80 μm, the chip area is 120000μm² and the area of the p-side electrode pad is 17600 μm². Thus, thearea of the p-side electrode pad accounts for about 15% of the chiparea.

FIG. 3 is a schematic plan view illustrating a shape of the p-sideelectrode pad of chip (A) in more detail. In this drawing, a nitridesemiconductor laser device 301 is set to have a chip width 302 of 50 to250 μm (e.g. 200 μm) and a length 303 of 300 to 1500 μm (e.g., 600 μm).A ridge-like stripe 304 is formed at a position spaced by a distance 306of 120 μm from the left side of the chip. A p-side electrode pad 305 isformed being retracted by a distance of 10 to 50 μM (e.g., 20 μm) fromthe chip division planes and the resonator end faces. For example, whenthe chip has a width of 200 μm and a length of 600 μm, and the p-sideelectrode pad is formed to have a width of 160 μm and a length of 560μm, the chip area is 120000 μm² and the area of the p-side electrode padis 89600 μm². Thus, the area of the p-side electrode pad accounts forabout 75% of the chip area.

In the first embodiment, therefore, the area ratio of the p-sideelectrode pad to the chip area in chip (B) accounts for 20%(=15÷75×100%) of that in chip (A).

FIG. 4 is a conceptual block diagram showing a general constitution ofan automatic chip inspection apparatus. A chip to be introduced into theautomatic chip inspection apparatus is set in a chip supplying area 401.Here, the automatic chip inspection apparatus recognizes the chip byautomatic image recognition, confirms its shape and carry out alignmentthereof, and then transfers the chip to a chip inspection area 402.After measurement of properties such as I-L and I-V properties of thesemiconductor laser in the chip inspection area, the automatic chipinspection apparatus transfers the measured chip to a chip storage area403.

In the image recognition in chip supplying area 401, an image of a chipis taken from its upper surface with a camera, and the shape of thep-side electrode pad of the laser device can be recognized based onbrightness distribution in the image. When a recognized size of thep-side electrode pad is compared with a size of a recognition imageinput beforehand into the automatic chip inspection apparatus andmatches the size of the recognition image within a tolerance inputbeforehand, the chip is transferred to the subsequent chip inspectionarea. On the other hand, if the recognized size of the p-side electrodepad falls out of the tolerance, the chip is determined as unacceptableand disregarded, and the automatic chip inspection apparatus shifts toimage recognition for a subsequent chip.

If the tolerance is set for example to 30% on this occasion, all ofchips (A) can be transported to chip inspection area 402. However, chip(B) is determined as unacceptable because the area of its p-sideelectrode pad falls out of the tolerance, differently from chip (A).Therefore, chip (B) is left in chip supplying area 401 without beingtransported to chip inspection area 402. Consequently, chips (B) left inchip supplying area 401 can be handled after chips (A) set in chipsupplying area 401 are all inspected and transferred to chip storagearea 403. That is, chip (A) and chip (B) can readily be separated basedon the shapes of their p-side electrode pads.

As described above, since chip (A) and chip (B) can be separated andinspected by the automatic chip inspection apparatus, it is possible toset inspection conditions optimized for each position of theirridge-like stripes. Further, since chip (A) and chip (B) can beseparately introduced into an automatic mounting apparatus, it ispossible to set process conditions optimized for each position of theirlight-emitting points.

In order to separate both the types of chips in a more stable manner, itis preferable to set the area ratio of the p-side electrode pad to thechip area in chip (B) to a value obtained by subtracting a tolerancefrom the area ratio in chip (A) or further subtracting more than 5%(preferably more than 10%). For example, when the area ratio of thep-side electrode pad to the chip area in chip (A) is 100% and theaforementioned tolerance is 0%, both the types of chips can be separatedin a stable manner if the area ratio of the p-side electrode pad to thechip area in chip (B) is set to 90%. In other words, in order to obtainthe effect of the present invention, the upper limit of the area ratioof the p-side electrode pad to the chip area is calculated as 90%, basedon the above calculation.

In order to separate both the types of chips in a more stable manner,however, the tolerance is preferably set to about ±1% to 50%, morepreferably at 10% to 40%. This is because, even when chips arefabricated with the same design, some chips are liable to be determinedas unacceptable due to contamination on the chip, peeling of the p-sideelectrode pad caused during the process, or scratches on the chip, evenif these defects cause no problems in the properties of the laserdevice. Consequently, when the aforementioned tolerance is set to 30%for example, the area ratio of the p-side electrode pad to the chip areain chip (B) is desirably set to less than 60% and preferably less than40% of the area ratio in chip (A).

FIG. 8 is a schematic cross-sectional view showing a main part of alayered structure in the nitride semiconductor laser device of the firstembodiment. This laser device includes an n-type GaN layer (thickness: 3μm) 802; an n-type In_(0.05)Ga_(0.95)N buffer layer 803; an n-typeAl_(0.05)Ga_(0.95)N clad layer 804 (thickness: 2.0 μm); an n-type GaNoptical waveguide layer 805 (thickness: 0.1 μm); anIn_(0.2)Ga_(0.8)N/n-type In_(0.05)Ga_(0.95)N (respective thicknesses: 4nm/8 nm) triple-quantum-well (3MQW) active layer 806; a p-typeAl_(0.2)Ga_(0.8)N carrier stop layer 807 (thickness: 20 nm); a p-typeGaN optical waveguide layer 808 (thickness: 0.1 μm); a p-typeAl_(0.05)Ga_(0.95)N clad layer 809 (thickness: 0.5 μm); and a p-type GaNcontact layer 810 (thickness: 0.2 μm) stacked in this order on an n-typeGaN substrate 801.

A ridge-like stripe 812 of 2 μm width is formed by partially etchingp-type GaN contact layer 810 through to a partial depth of p-type AlGaNclad layer 809. The laser device includes an optical confinementwaveguide structure in which active layer 806 and optical waveguidelayers 805 and 808 are sandwiched between clad layers 804 and 809, andlight generated in the active layer is confined in this waveguidestructure and causes lasing. It should be noted that FIG. 8 does notshow the entire width of the laser device chip but only a part thereofin the vicinity of the ridge-like stripe.

A p-side contact electrode 811 is formed on p-type GaN contact layer 810left after the partial etching, while an insulating film 813 and a metallayer 816 of less than 0.1 μm thickness are formed on the partiallyetched areas. A p-side electrode pad 814 is formed to cover p-sidecontact electrode 811. In the first embodiment, metal layer 816 isformed to improve adhesiveness between insulating film 813 and p-sideelectrode pad 814; by preferably using Mo or the like, though therearises no problem in laser device properties even if it is omitted.

On the other hand, an n-side electrode 815 is formed on a rear surfaceof substrate 801. The laser device of FIG. 8 is obtained by dividing awafer including a large number of device structures into chips. Tofacilitate the chip division, therefore, substrate 801 is polished tohave a thickness of 50 to 200 μm before n-side electrode 815 is formedthereon. Then, the wafer is divided into chips to obtain the nitridesemiconductor laser device of FIG. 8 with a width of 300 to 400 μm.

As described above, it is necessary in the present invention torecognize the shape of the p-side electrode pad in the nitridesemiconductor laser device by automatic image recognition. On the uppersurface of the chip, therefore, as compared to the area having thep-side electrode pad, the other remaining area should have a lowerreflectance with respect to almost the entire wavelength range ofincident illumination light vertical to the chip during the automaticimage recognition. Here, if metal layer 816 has a thickness of more than0.1 μm, there is only a small difference in reflectance between the areahaving p-side electrode pad 814 and the other remaining area. Incontrast, if metal layer 816 has a thickness of less than 0.1 μm, thenormal incident light transmits through metal layer 816, and thus it ispossible to cause a difference in reflectance between the area havingp-side electrode pad 814 and the other remaining area. That is, althoughthe effect of the present invention can be obtained even without metallayer 816, if metal layer 816 is provided to improve adhesiveness ofp-side electrode pad 814 to insulating film 813, it is desirable thatmetal layer 816 has a thickness of less than 0.1 μm.

Since the problems to be solved by the present invention are caused whenthe chip has a relatively narrow width such as 50 to 250 μm as describedabove, the present invention is particularly effective when there is aneed to reduce the chip width. Although the ridge-like stripe is formedat a position 120 μm away from the right or left side of the chip in thefirst embodiment, it should be noted that it may be formed at anotherposition as long as it is spaced from the trenched region of thesubstrate by more than 90 μm, preferably by more than 110 μm. Further,although the chip division plane is provided along the trenched regionin the first embodiment, this is not necessary.

Second Embodiment

FIG. 5 is a schematic plan view illustrating a shape of a p-sideelectrode pad of a chip (B) as a nitride semiconductor laser deviceaccording to a second embodiment of the present invention. In thisdrawing, a nitride semiconductor laser device 501 is set to have a width502 of 50 to 250 μm (e.g., 200 μm) and a length (corresponding to aresonator length) 503 of 300 to 1500 μm (e.g., 600 μm). A trenchedregion 504 of a worked substrate exists along the right side of chip501. A ridge-like stripe 505 is formed at a position spaced by adistance 508 of 120 μm from the right side of the chip. A p-sideelectrode pad 506 is formed to include its partial area placed to coverridge-like stripe 505 and another partial area (e.g., a square with aside of 80 μm) including a circle 507 of 80 μm diameter for wirebonding. P-side electrode pad 506 is formed with its area falling in arange of more than 2% and less than 90% of the chip area.

Circular area 507 for wire bonding is placed preferably more than 5 μmand more preferably more than 10 μm (e.g., 20 μm) away from ridge-likestripe 505. This structure can eliminate a risk that ultrasonic powerapplied during the wire bonding may reach a semiconductor part near theridge-like stripe and cause defects such as cracks.

In the second embodiment, for example, when chip 501 has a width of 250μm and a length of 1500 μm, and p-side electrode pad 506 includes itspartial area with a width extending by 20 μm to each of the right andleft sides from the center of ridge-like stripe 505 and its anotherpartial area of a square with a side of 80 μm including circle 207 of 80μm diameter for wire bonding, the chip area is 375000 μm² and the areaof the p-side electrode pad is 9600 μm². Thus, the area of the p-sideelectrode pad accounts for about 2.5% of the chip area. In order toobtain the effect of the present invention, the lower limit of the arearatio of the p-side electrode pad to the chip area is 2.5% as calculatedby the above calculation.

Third Embodiment

FIG. 6 is a schematic cross-sectional view showing a main part of alayered structure in a nitride semiconductor laser device according to athird embodiment of the present invention. The cross-sectional structureof FIG. 6 is different from that of FIG. 8 only in that metal layer 816of less than 0.1 μm thickness is replaced by a layer 616 of a lightabsorbing material of more than 10 nm thickness. That is, other layers601 to 615 in FIG. 6 correspond to layers 801 to 815 in FIG. 8,respectively. Further, the p-side electrode pad in the nitridesemiconductor laser device of the third embodiment can have a shapeidentical to that of the first or second embodiment.

Since light absorbing layer 616 is formed on insulating film 613 in thenitride semiconductor laser device of the third embodiment, an areahaving p-side electrode pad 614 has a higher reflectance with respect tovisible light, as compared to the other remaining area not having p-sideelectrode pad 614. Owing to this, when image recognition is carried outin the automatic chip inspection apparatus, the high contrast betweenthe p-side electrode pad and the other part makes it possible to obtainan effect that recognition errors during the image recognition cansufficiently be prevented.

To make this effect significant, it is desirable during the automaticimage recognition that, as compared to the area having the p-sideelectrode pad, the area not having the p-side electrode pad is lower bymore than 10% (preferably, by more than 20%) in reflectance with respectto almost the entire wavelength range of incident illumination lightvertical to the chip. This is achieved if light absorbing layer 616 hasan absorption coefficient of more than 10000 cm⁻¹. As a materialsatisfying this condition, Si, Ge, TiO₂, or the like can be used forexample. Desirably, light absorbing layer 616 also serves as aninsulating layer, and for this purpose, light absorbing layer 616 ispreferably an insulator such as TiO₂ or a semiconductor such as Si orGe.

Fourth Embodiment

A nitride semiconductor laser device according to a fourth embodiment ofthe present invention is different from those of the first and secondembodiments only in that it has minute surface unevenness on bottomsurfaces of areas partially etched and removed to form the ridge-likestripe. With such surface unevenness, as compared to the area having thep-side electrode pad, the area not having the p-side electrode pad canbe lower by more than 10% in reflectance with respect to almost theentire wavelength range of incident illumination light vertical to thechip during the automatic image recognition. The minute surfaceunevenness can realize the above reflectance ratio less by more than 10%when the RMS (root mean square) roughness is more than 1 nm over alength of 5 μm in a direction parallel to the etched bottom surface. Theminute surface unevenness can be formed with adjustment of etchingconditions as appropriate.

Fifth Embodiment

In a fifth embodiment of the present invention, one of the nitridesemiconductor laser devices disclosed in the aforementioned embodimentsis used as a light source for reproduction in an optical informationreproducing apparatus. The optical information reproducing apparatus caninclude known components as components other than the light source.

FIG. 7 is a schematic block diagram illustrating an optical system and apropagation path of light for reproduction in an optical informationreproducing apparatus according to the fifth embodiment of the presentinvention. This optical information reproducing apparatus includes anitride semiconductor laser device 701 of the present invention, beamemission control means (not shown), a collimator lens 702, a beamshaping prism 703, a beam splitter 704, an objective lens (focusingmeans) 705, an optical disk (optical recording medium) 706, focusposition control means (not shown), and a light detecting system (lightdetecting means) 707 for detecting light. For clarity and simplicity ofthe drawing, components (means) not important for illustrating thefeatures of the present invention are omitted in FIG. 7. Needless tosay, on the other hand, optical disk (optical recording medium) 706shown in FIG. 7 is not specific to the optical information reproducingapparatus and is accepted when information is recorded or reproduced.

In the optical information reproducing apparatus of the fifthembodiment, nitride semiconductor laser device 701 can serve as a lightsource for both recording and reproduction. In recording operation anderasing operation, laser light emitted from nitride semiconductor laserdevice 701 is converted to parallel light or nearly parallel light bycollimator lens 702, passes through beam splitter 704, and is focused byobjective lens 705 onto an information recording surface of optical disk706. Then, bit-information is written on the recording surface ofoptical disk 706 by magnetic modulation or refractive index modulation.In reproducing operation, laser light emitted from nitride semiconductorlaser device 701 is focused onto the record surface of optical disk 706recorded by unevenness, magnetic modulation, or refractive indexmodulation. The focused laser light is reflected by the informationrecord surface, passes through objective lens 705 and beam splitter 704,and then enters light detecting system 707 in which an opticallydetected signal is converted to an electrical signal to read recordedinformation.

Since the optical information reproducing apparatus of the fifthembodiment uses a nitride semiconductor laser device that is inexpensiveand has a small chip width in a range of 50 to 250 μm, its productioncost can be suppressed.

As has been described, the present invention can provide a nitridesemiconductor laser device of a small width at a low cost, and alsocontribute to cost reduction of an optical information reproducingapparatus by using the laser device.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

1. A method of producing a nitride semiconductor laser device,comprising: forming a wafer including a nitride semiconductor layer of afirst conductivity type, an active layer of a nitride semiconductor, anitride semiconductor layer of a second conductivity type, and anelectrode pad for the second conductivity type stacked in this order ona main surface of a conductive substrate, said wafer also including aplurality of stripe-like waveguide structures parallel to said activelayer; cutting said wafer to obtain a first type of nitridesemiconductor laser device chip and a second type of nitridesemiconductor laser device chip; and distinguishing between said firsttype and second type of nitride semiconductor laser device chips byautomatic image recognition, wherein said first type and second type ofnitride semiconductor laser device chips differ from each other as topositions of said plurality of stripe-like waveguide structures withrespect to a width direction of each of said first type and second typeof nitride semiconductor laser device chips and as to an area ratio ofsaid electrode pad for the second conductivity type to said main surfaceof said conductive substrate.
 2. The method of producing a nitridesemiconductor laser device according to claim 1, wherein the area ratioof said electrode pad for the second conductivity type in said firsttype of nitride semiconductor laser device chip is less than 60% of thearea ratio of said electrode pad for the second conductivity type insaid second type of nitride semiconductor laser device chip.
 3. Themethod of producing a nitride semiconductor laser device according toclaim 1, further comprising: a metal layer of less than 0.1 μm thicknesson an upper surface of said nitride semiconductor layer of the secondconductivity type in an area not having said electrode pad for thesecond conductivity type in each of said first type and second type ofnitride semiconductor laser device chips.
 4. The method of producing anitride semiconductor laser device according to claim 1, wherein saidelectrode pad for the second conductivity type includes a circular areaof 80 μm diameter in each of said first type and second type of nitridesemiconductor laser device chips.
 5. The method of producing a nitridesemiconductor laser device according to claim 4, wherein said circulararea is spaced by a distance of more than 10 μm from said stripe-likewaveguide structure in each of said first type and second type ofnitride semiconductor laser device chips.
 6. The method of producing anitride semiconductor laser device according to claim 1, wherein saidmain surface of said conductive substrate is a rectangle having twosides parallel to and the other two sides perpendicular to saidstripe-like waveguide structure, and each of said other two sides has alength of more than 50 μm and less than 250 μm in each of said firsttype and second type of nitride semiconductor laser device chips.
 7. Themethod of producing a nitride semiconductor laser device according toclaim 1, wherein an area not having said electrode pad for the secondconductivity type on an upper surface of said nitride semiconductorlayer of the second conductivity type in each of said first type andsecond type of nitride semiconductor laser device chips is lower by morethan 10% in reflectance with respect to almost the entire wavelengthrange of incident illumination light vertical to said first type andsecond type of nitride semiconductor laser device chips compared to anarea having said electrode pad for the second conductivity type duringsaid automatic image recognition.
 8. The method of producing a nitridesemiconductor laser device according to claim 7, wherein the area nothaving said electrode pad for the second conductivity type on the uppersurface of said nitride semiconductor layer of the second conductivitytype has a layer of more than 10 nm thickness made of an absorbingmaterial having an absorption coefficient of more than 10000 cm⁻¹ withrespect to almost the entire wavelength range of incident illuminationlight vertical to said first type and second type of nitridesemiconductor laser device chips during said automatic imagerecognition.
 9. The method of producing a nitride semiconductor laserdevice according to claim 8, wherein said absorbing material is aninsulator or a semiconductor.
 10. The method of producing a nitridesemiconductor laser device according to claim 8, wherein said absorbingmaterial includes silicon, germanium, or TiO₂.
 11. The method ofproducing a nitride semiconductor laser device according to claim 7,wherein the area not having said electrode pad for the secondconductivity type on the upper surface of said nitride semiconductorlayer of the second conductivity type has minute surface unevenness, anda root mean square roughness over a length of 5 μm in a directionparallel to the uneven surface is more than 1 nm and less than 200 nm ineach of said first type and second type of nitride semiconductor laserdevice chips.